Method for improving the ozone stability of an inkjet recording element

ABSTRACT

The present invention relates to a method for improving the ozone stability of an inkjet recording element. The present invention consists in using at least one sulfonic polystyrene or one of its sulfonate salts in an inkjet recording element, said element comprising a support and at least one ink-receiving layer including at least one hydrosoluble binder and inorganic fillers, for improving the ozone stability of said element.

FIELD OF THE INVENTION

The present invention relates to a method for improving the ozonestability of an inkjet recording element.

Digital photography has been growing fast for several years and thegeneral public now has access to efficient and reasonably priced digitalcameras. Therefore people are seeking to be able to produce photographicprints from a simple computer and its printer, with the best possiblequality.

Many printers, especially those linked to personal office automation,use the inkjet printing technique. There are two major families ofinkjet printing techniques: continuous jet and drop-on-demand.

Continuous jet is the simpler system. Pressurized ink (3.10⁵ Pa) isforced to go through one or more nozzles so that the ink is transformedinto a flow of droplets. In order to obtain the most regular possiblesizes and spaces between drops, regular pressure pulses are sent usingfor example a piezoelectric crystal in contact with the ink with highfrequency (up to 1 MHz) alternating current (AC) power supply. So that amessage can be printed using a single nozzle, every drop must beindividually controlled and directed. Electrostatic energy is used forthis: an electrode is placed around the ink jet at the place where dropsform. The jet is charged by induction and every drop henceforth carriesa charge whose value depends on the applied voltage. The drops then passbetween two deflecting plates charged with the opposite sign and thenfollow a given direction, the amplitude of the movement beingproportional to the charge carried by each of the plates. To preventother drops from reaching the paper, they are left uncharged: so,instead of going to the support they continue their path without beingdeflected and go directly into a container. The ink is then filtered andcan be reused.

The other category of inkjet printer is drop-on-demand (DOD). Thisconstitutes the base of inkjet printers used in office automation. Withthis method, the pressure in the ink cartridge is not maintainedconstant but is applied when a character has to be formed. In onewidespread system there is a row of 12 open nozzles, each of them beingactivated with a piezoelectric crystal. The ink contained in the head isgiven a pulse: the piezo element contracts with an electric voltage,which causes a decrease of volume, leading to the expulsion of the dropby the nozzle. When the element resumes its initial shape, it pumps inthe reservoir the ink necessary for new printings. The row of nozzles isthus used to generate a column matrix, so that no deflection of the dropis necessary. One variation of this system consists in replacing thepiezoelectric crystals by small heating elements behind each nozzle. Thedrops are ejected following the forming of bubbles of solvent vapor. Thevolume increase enables the expulsion of the drop. Finally, there is apulsed inkjet system in which the ink is solid at ambient temperature.The print head thus has to be heated so that the ink liquefies and canprint. This enables rapid drying on a wider range of products thanconventional systems.

There now exist new “inkjet” printers capable of producing photographicimages of excellent quality. However, they cannot supply good proofs ifinferior quality printing paper is used. The choice of printing paper isfundamental for the quality of obtained image. The printing paper mustcombine the following properties: a printed image of high quality, rapiddrying after printing, a smooth glossy appearance, and good resistanceof the image colors in time, which means especially good stability toozone present in the atmosphere of the ink dyes.

In general, the printing paper comprises a support coated with one ormore layers according to the properties required. Two main technologieshave been developed. On the one hand, there is a non-porous printingpaper, usually comprising layers of polymers, such as gelatin. Thispaper enables images that are glossy and ozone stable to be obtainedbecause, once the polymer layer is dry, permeability to ozone is low.However, these papers, not being porous, have to swell to absorb theink. This swelling slows ink absorption so that the ink can easily runjust after printing.

Another paper has been developed to obtain a paper with rapid drying inorder to increase printing output rates. This is a paper comprising aporous ink-receiving layer including colloidal particles used asreceiving agent and a polymer binder. This porous paper absorbs the inkrapidly thanks to the pores existing between the particles.

The purpose of the receiving agent is to fix the dyes in the printingpaper. The best-known inorganic receivers are colloidal silica orboehmite. For example, the European Patent Applications EP-A-976,571 andEP-A-1,162,076 describe materials for inkjet printing in which theink-receiving layer contains as inorganic receivers Ludox™ CL (colloidalsilica) marketed by Grace Corporation or Dispal™ (colloidal boehmite)marketed by Sasol. However, printing paper comprising a porousink-receiving layer can have poor ozone stability in time, which isdemonstrated by a loss of color density. In particular this is due tothe fact that the colloidal particles are easily accessible to ozone andthe surface of these particles could catalyze the ozone degradation ofthe ink dyes.

To meet new market needs in terms of printing speed and color stabilityto ozone, it is necessary to propose a method enabling improvement ofthe ozone stability of a material intended for inkjet printing.

SUMMARY OF THE INVENTION

The present invention relates to the use of at least one sulfonicpolystyrene or one of its sulfonate salts in an inkjet recordingelement, said element comprising a support and at least oneink-receiving layer including at least one hydrosoluble binder andinorganic fillers, for improving the ozone stability of said element.

In preference, one uses sulfonic polystyrene in salt form chosen fromamong the group consisting of sodium polystyrene sulfonate, lithiumpolystyrene sulfonate, ammonium polystyrene sulfonate, and potassiumpolystyrene sulfonate.

The use of sulfonic polystyrene or polystyrene sulfonate enables theozone stability of a porous inkjet recording element to be improved forinkjet printing while obtaining an element having at least the same inkinstant-dryness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, 4 to 8, and 10 to 14 represent the percentage of colordensity loss for various elements exposed to ozone, and

FIGS. 3 and 9 represent the percentage of instant dryness for variouselements.

DETAILED DESCRIPTION OF THE INVENTION

An inkjet recording element comprises firstly a support. This support isselected according to the desired use. It can be a transparent or opaquethermoplastic film, in particular a polyester base film such aspolyethylene terephthalate; cellulose derivatives, such as celluloseester, cellulose triacetate, cellulose diacetate; polyacrylates;polyimides; polyamides; polycarbonates; polystyrenes; polyolefines;polysulfones; polyetherimides; vinyl polymers such as polyvinylchloride; and their mixtures. The support used in the invention can alsobe paper, both sides of which may be covered with a polyethylene layer.When the support comprising the paper pulp is coated on both sides withpolyethylene, it is called Resin Coated Paper (RC Paper) and is marketedunder various brand names. This type of support is especially preferredto constitute an inkjet recording element. The side of the support thatis used can be coated with a very thin layer of gelatin or anothercomposition to ensure the adhesion of the first layer on the support. Toimprove the adhesion of the ink-receiving layer on the support, thesupport surface can also have been subjected to a preliminary treatmentby Corona discharge before applying the ink-receiving layer.

An inkjet recording element then comprises at least one ink-receivinglayer including at least one hydrosoluble binder. Said hydrosolublebinder can be a hydrophilic polymer such as polyvinyl alcohol,poly(vinyl pyrrolidone), gelatin, cellulose ethers, poly(oxazolines),poly(vinylacetamides), poly(vinyl acetate/vinyl alcohol) partiallyhydrolyzed, poly(acrylic acid), poly(acrylamide), sulfonated orphosphated polyesters and polystyrenes, casein, zein, albumin, chitin,dextran, pectin, derivatives of collagen, agar-agar, guar,carragheenane, tragacanth, xanthan and others. In preference, one usesgelatin or polyvinyl alcohol. The gelatin is that conventionally used inthe photographic field. Such a gelatin is described in ResearchDisclosure, September 1994, No. 36544, part IIA. Research Disclosure isa publication of Kenneth Mason Publications Ltd., Dudley House, 12 NorthStreet, Emsworth, Hampshire PO10 7DQ, United Kingdom. The gelatin can beobtained from SKW and the polyvinyl alcohol from Nippon Gohsei, or AirProduct with the name Airvol® 130.

The ink-receiving layer also comprises, as receiving agent, inorganicfillers. In one embodiment, said inorganic fillers are based on metaloxide or metal hydroxide. In preference, the inorganic fillers are basedon alumina, silica, titanium, zirconium, or their mixtures. Preferably,the inorganic fillers are chosen from among the group consisting of theboehmites, fumed aluminas, colloidal silicas, fumed silicas, calciumsilicates, magnesium silicates, zeolites, kaolin, bentonite, silicondioxide, and titanium dioxide. According to another embodiment, theinorganic fillers are based on calcium carbonates, or baryum carbonates.

The quantities of the inorganic fillers and hydrosoluble binder arethose generally used for porous inkjet recording elements.

Those skilled in the art know that such inorganic fillers, used alone asa receiving agent in ink-receiving layers, do not enable the requiredozone stability of the image to be obtained for inkjet recordingelements.

According to the present invention, the use of at least one sulfonicpolystyrene or polystyrene sulfonate salt enables the ozone stability ofthe dyes making up the image to be improved.

Such sulfonic polystyrenes or polystyrene sulfonates are polymersmarketed for example by Alco Chemical™ under the VERSA-TL® brand, andcan have a molecular weight between 15,000 and 1,000,000.

When polystyrene sulfonate is used, it can be chosen from among thegroup consisting of sodium, lithium, ammonium and potassium polystyrenesulfonate.

The quantity of sulfonic polystyrene or polystyrene sulfonate (in drystate) is between 0.1% and 10% by weight with reference to the totalweight of the wet receiving layer. Preferably, the quantity of sulfonicpolystyrene or polystyrene sulfonate (in dry state) is between 0.25% and8% by weight with reference to the total weight of the wet receivinglayer. Still more preferably, the quantity of sulfonic polystyrene orpolystyrene sulfonate (in dry state) is between 1% and 3% by weight withreference to the total weight of the wet receiving layer.

The sulfonic polystyrene or polystyrene sulfonate is added to the layercomposition intended to be coated on the support to constitute theink-receiving layer of the element described above. To produce thiscomposition, in preference, the sulfonic polystyrene or polystyrenesulfonate in the form of an aqueous solution and the inorganic fillersare first mixed together and then the hydrosoluble binder is added. Thecomposition then has the form of an aqueous solution or a dispersioncontaining all the necessary components.

The composition can be layered on the support according to anyappropriate coating method, such as blade, knife or curtain coating. Thecomposition is applied with a thickness between approximately 100 μm and200 μm in the wet state. The ink-receiving layer has a thickness usuallybetween 5 μm and 50 μm in the dry state. The composition forming theink-receiving layer can be applied on both sides of the support. It isalso possible to provide an antistatic or anti-winding layer on the backof the support coated with the ink-receiving layer.

The inkjet recording element can comprise, besides the ink-receivinglayer described above, other layers having another function, arrangedabove or below said ink-receiving layer. The ink-receiving layer as wellas the other layers can comprise all the other additives known to thoseskilled in the art to improve the properties of the resulting image,such as surface-actives, UV ray absorbers, optical brightening agents,antioxidants, plasticizers, etc.

The use of sulfonic polystyrene or polystyrene sulfonate in an inkjetrecording element comprising an ink-receiving layer including inorganicfillers enables the ozone stability of the ink dyes to be improved,without degrading the element drying properties. This invention can beused for any type of inkjet printer as well as for all the inksdeveloped for this technology.

The following examples illustrate the present invention without howeverlimiting the scope.

1) Preparing Compositions Intended to be Layered on a Support toConstitute an Ink-Receiving Layer

As hydrosoluble binder, polyvinyl alcohol was used (Gohsenol™ GH23marketed by Nippon Gohsei) diluted to 9% in osmosis water.

The polystyrene was used in the form of two sulfonate salts as given inTable I below and marketed by Alco Chemical™: TABLE I PolystyreneMolecular sulfonate Salt Form weight Versa TL ® 502 Sodium salt solid1,000,000 Versa TL ® 73 Lithium salt Solution at 70,000 30% in water

The inorganic fillers used are given below in Table II: TABLE IIInorganic filler Brand Charge Supplier Fumed alumina CAB-O-SPERSE ®Positive Cabot (dispersion at 40%) PG003 Corporation Colloidal silicaOndeo Nalco ®2329 Negative Ondeo Nalco (dispersion at 40%) CorporationColloidal silica Ludox ™ PGE Negative Grace (dispersion at 30%)Corporation

All the compositions resulted from mixing:

0 g to 5 g dry polystyrene sulfonate (see Table III below)

13.5 g dry inorganic fillers

18.1 g polyvinyl alcohol in aqueous solution at 9%

Water to 100 g

First the mixture of polystyrene sulfonate in aqueous solution and theinorganic fillers was made in a glass bottle comprising glass beads withdiameter 10 mm. The mixture was stirred for three hours on a rollerstirrer. Then the polyvinyl alcohol was added and stirred for 18 hourson a roller stirrer. Before being layered, the resulting mixture was putinto a tank thermostatically controlled to 50° C. to lower the viscosityof said mixture.

2) Preparing Inkjet Recording Elements

To do this, a Resin Coated Paper type support was placed on a coatingmachine, first coated with a very thin gelatin layer, and held on thecoating machine by vacuum. This support was coated with a composition asprepared according to paragraph 1 using a filmograph to obtain a wetthickness of 200 μm. Then, it was left to dry overnight at ambient airtemperature (21° C.).

The resulting materials correspond to the examples shown in Table IIIbelow giving the polystyrene sulfonate and the quantity used in theink-receiving layer, as well as the inorganic filler used: TABLE III %by Inorganic filler Polystyrene weight of added to the ink- sulfonatedry PSS in Material receiving layer (PSS) the mixture Ex 1 (comp.)Colloidal silica Versa TL ® 502 0 Ludox ™ PGE Ex 2 (inv.) Colloidalsilica Versa TL ® 502 0.25 Ludox ™ PGE Ex 3 (inv.) Colloidal silicaVersa TL ® 502 1.25 Ludox ™ PGE Ex 4 (inv.) Colloidal silica Versa TL ®502 2.5 Ludox ™ PGE Ex 5 (inv.) Colloidal silica Versa TL ® 502 5Ludox ™ PGE Ex 6 (comp.) Colloidal silica Versa TL ® 73 0 Ludox ™ PGE Ex7 (inv.) Colloidal silica Versa TL ® 73 0.25 Ludox ™ PGE Ex 8 (inv.)Colloidal silica Versa TL ® 73 1.25 Ludox ™ PGE Ex 9 (inv.) Colloidalsilica Versa TL ® 73 2.5 Ludox ™ PGE Ex 10 (inv.) Colloidal silica VersaTL ® 73 5 Ludox ™ PGE Ex 11 (comp.) Colloidal silica Versa TL ® 502 0Nalco ®2329 Ex 12 (inv.) Colloidal silica Versa TL ® 502 0.25Nalco ®2329 Ex 13 (inv.) Colloidal silica Versa TL ® 502 0.5 Nalco ®2329Ex 14 (inv.) Colloidal silica Versa TL ® 502 1 Nalco ®2329 Ex 15 (inv.)Colloidal silica Versa TL ® 502 3 Nalco ®2329 Ex 16 (comp.) Colloidalsilica Versa TL ® 73 0 Nalco ®2329 Ex 17 (inv.) Colloidal silica VersaTL ® 73 0.25 Nalco ®2329 Ex 18 (inv.) Colloidal silica Versa TL ® 73 0.5Nalco ®2329 Ex 19 (inv.) Colloidal silica Versa TL ® 73 1 Nalco ®2329 Ex20 (inv.) Colloidal silica Versa TL ® 73 3 Nalco ®2329 Ex 21 (comp.)Fumed alumina Versa TL ® 502 0 CAB-O-SPERSE ® PG003 Ex 22 (inv.) Fumedalumina Versa TL ® 502 0.25 CAB-O-SPERSE ® PG003 Ex 23 (inv.) Fumedalumina Versa TL ® 502 0.5 CAB-O-SPERSE ® PG003 Ex 24 (inv.) Fumedalumina Versa TL ® 502 1 CAB-O-SPERSE ® PG003 Ex 25 (comp.) Fumedalumina Versa TL ® 73 0 CAB-O-SPERSE ® PG003 Ex 26 (inv.) Fumed aluminaVersa TL ® 73 0.25 CAB-O-SPERSE ® PG003 Ex 27 (inv.) Fumed alumina VersaTL ® 73 0.5 CAB-O-SPERSE ® PG003 Ex 28 (inv.) Fumed alumina Versa TL ®73 1 CAB-O-SPERSE ® PG003 Ex 29 (inv.) Fumed alumina Versa TL ® 73 3CAB-O-SPERSE ® PG0033) Evaluating the Ozone Stability of the Image over Time

To evaluate ozone stability over time, a color alteration test byexposure to ozone was performed for each resulting inlet recordingelement. For this, targets composed of four colors, cyan, magenta,yellow and black were printed on each element using a KODAK® PPM 200printer and related ink and/or an Epson® 670 or Epson® 890 printer andrelated inks, to test equally the various dyes in the inks. The targetswere analyzed using a Gretag Macbeth Spectrolino densitometer thatmeasured the intensity of the various colors. Then the elements wereplaced to the dark in a room with controlled ozone atmosphere (60 ppb)for several weeks. Each week, any degradation of the color density wasmonitored using the densitometer.

4) Evaluating the Instant Dryness

Also measured for certain resulting elements was the instant dryness ofthe ink once printed on said elements.

For this, a target made of 10-cm long strips of four colors, cyan,magenta, yellow and black, was printed on certain of the resultingelements. Just at the end of printing, a sheet of plain paper was putonto the freshly printed target and pressure was applied using a smoothheavy roller. The sheet of plain paper was then removed and theintensity of the transferred image of the target was measured on saidsheet. The more intense the transferred image was, the slower theinstant dryness of the ink was.

The instant dry percentage is defined by the following equation:${\%\quad{Dry}} = {\left( {1 - \frac{1 - 10^{- {Dmes}}}{1 - 10^{- {{Dref}.}}}} \right) \times 100}$

where D_(mes) is the optical density measured on the image of thetransferred target less D_(min) (density of the resulting element notprinted)

-   -   D_(ref) is the optical density measured on a target printed on        the sheet of plain paper less D_(min).

The optical densities were measured using the Gretag Macbeth Spectrolinodensitometer.

The change in the instant dry percentage is linked to the instantdryness of the ink. The higher the percentage % Dry is, the better theinstant dryness of the ink is.

5) Results

FIG. 1 represents the percentage of density loss observed for themaximum density for the four colors of the target after three weeks forexamples 1 to 5 printed using the Kodak® PPM200 printer. Letters C, M, Yand K represent the colors cyan, magenta, yellow and black respectively.

FIG. 2 represents the percentage of density loss observed for themaximum density for the four colors of the target after three weeks forexamples 1 to 5 printed using the Epson® 890 printer.

FIG. 3 represents the instant dry percentage % Dry for the colors, cyan,magenta and black, for the examples 1 to 5 printed using the Epson® 890printer.

FIG. 4 represents the percentage of density loss observed for themaximum density for the four colors of the target after four weeks forexamples 6 to 10 printed using the Kodak® PPM200 printer.

FIG. 5 represents the percentage of density loss observed for themaximum density for the four colors of the target after four weeks forexamples 6 to 10 printed using the Epson® 670 printer.

These FIGS. 1 to 5 show that the use of polystyrene sulfonate (Versa TL®502 or Versa TL® 73), in an inkjet recording element comprising anink-receiving layer including colloidal silica Ludox™ PGE as inorganicreceiving agent, enabled the ozone stability of the dyes to be increasedconsiderably, even when very little polystyrene sulfonate was used.

Further, FIG. 3 shows that the instant dry percentage is alsoconsiderably increased. The use of polystyrene sulfonate did not degradethe instant dryness of the ink but, quite the opposite, enabled it to beimproved compared with the element not containing polystyrene sulfonate.

FIG. 6 represents the percentage of density loss observed for themaximum density for the four colors of the target after five weeks forexamples 11 to 15 printed using the Kodak® PPM200 printer.

FIG. 7 represents the percentage of density loss observed for themaximum density for the four colors of the target after five weeks forexamples 11 to 15 printed using the Epson® 670 printer.

FIG. 8 represents the percentage of density loss observed for themaximum density for the four colors of the target after three weeks forexamples 16 to 20 printed using the Kodak® PPM200 printer.

FIG. 9 represents the percentage of density loss observed for themaximum density for the four colors of the target after three weeks forexamples 16 to 20 printed using the Epson® 890 printer.

FIG. 10 represents the instant dry percentage % Dry for the colors,cyan, magenta and black, for the examples 16 to 20 printed using theEpson® 890 printer.

These FIGS. 1 to 6 show that the use of polystyrene sulfonate (Versa TL®502 or Versa TL® 73), in an inkjet recording element comprising anink-receiving layer including colloidal silica Nalco® 2329 as inorganicreceiving agent, enabled the ozone stability of the dyes to be increasedconsiderably, in particular when 1% to 3% polystyrene sulfonate wasused.

Further, FIG. 10 shows that the instant dry percentage of instantly dryink was also considerably increased when 1% to 3% polystyrene sulfonatewas used. The use of polystyrene sulfonate did not degrade the instantdryness of the ink but, quite the opposite, enabled it to be improvedcompared with the element not containing polystyrene sulfonate.

FIG. 11 represents the percentage of density loss observed for themaximum density for the four colors of the target after three weeks forexamples 21 to 24 printed using the Kodak® PPM200 printer.

FIG. 12 represents the percentage of density loss observed for themaximum density for the four colors of the target after three weeks forexamples 21 to 24 printed using the Epson® 890 printer.

FIG. 13 represents the percentage of density loss observed for themaximum density for the four colors of the target after three weeks forexamples 25 to 29 printed using the Kodak® PPM200 printer.

FIG. 14 represents the percentage of density loss observed for themaximum density for the four colors of the target after three weeks forexamples 25 to 29 printed using the Epson® 890 printer.

These FIGS. 11 to 14 show that the use of polystyrene sulfonate (VersaTL® 502 or Versa TL® 73), in an inkjet recording element comprising anink-receiving layer including fumed alumina as inorganic receivingagent, enabled the ozone stability of the dyes to be increased, inparticular when 1% to 3% polystyrene sulfonate was used.

1. Use of at least one sulfonic polystyrene or one of its sulfonate salts in an inkjet recording element, said element comprising a support and at least one ink-receiving layer including at least one hydrosoluble binder and inorganic fillers, for improving the ozone stability of said element.
 2. The use according to claim 1, wherein the polystyrene sulfonate is chosen from among the group consisting of sodium polystyrene sulfonate and lithium polystyrene sulfonate.
 3. The use according to claim 1, wherein the amount of sulfonic polystyrene or polystyrene sulfonate in the dry state is between 0.1% and 10% by weight compared with the total weight of the wet receiving layer.
 4. The use according to claim 3, wherein the amount of sulfonic polystyrene or polystyrene sulfonate in the dry state is between 1% and 3% by weight compared with the total weight of the wet receiving layer.
 5. The use according to claim 1, wherein the inorganic fillers are based on metal oxide or metal hydroxide.
 6. The use according to claim 5, wherein the inorganic fillers are based on alumina, silica, titanium, zirconium, or their mixtures.
 7. The use according to claim 6, wherein the inorganic fillers are chosen from among the group consisting of the boehmites, fumed aluminas, colloidal silicas, fumed silicas, calcium silicates, magnesium silicates, zeolites, kaolin, bentonite, silicon dioxide, and titanium dioxide.
 8. The use according to claim 1, wherein the inorganic fillers are based on calcium carbonates or baryum carbonates.
 9. The use according to claim 1, wherein the hydrophilic binder is gelatin or polyvinyl alcohol. 